Since the advent of shipping vessels especially adapted to carry cargo as previously loaded into ISO containers, many improvements have been made in efforts to enhance or facilitate the loading and stowage of these container. For the most part, such efforts have been directed toward the development of lashing devices and guide systems to permit the loading of greater numbers of containers and with less labor requirements. The side walls of containership hulls as heretofore constructed provide a freeboard which has been designated by the International Load Line Convention 1986 to meet the requirements calculated to satisfy safe operation in prescribed sea lanes. These parameters are directed toward insuring seaworthy operation under all anticipated conditions with the required freeboard serving to accommodate fluctuations in green water conditions. Thus, shipping regulations with respect to the load water line and minimum statutory freeboard will be understood to vary, not only for sea lanes in different geographical zones but also, for winter and summer operations within each of these zones. Given any one sea lane, the minimum statutory freeboard may vary from 0.8% to 1.5% of a containership's length between perpendiculars. In calculating this length, the two perpendiculars will be understood to comprise lines respectively passing: 1) through the ships's rudder post center line and the summer toad waterline and (2) the line passing through the bow and the summer load waterline.
Conventional containership frequently are constructed with an increased freeboard, over the minimum statutory freeboard, allowing for the loading of one or more additional tier of containers between the strength decks of the hold side walls. In any case, with or without this increased freeboard, hatch covers are required to be provided atop the tier of containers disposed between the strength decks to maintain the necessary seaworthy conditions. The hatch covers themselves are of heavy, robust construction, necessary to withstand the substantial acceleration forces due to the ship's motion in a seaway and wave impact loading. Also, since several additional tiers of containers are usually carried atop the installed hatch covers, it follows that the construction of the hatch covers must be significant enough to withstand the noteworthy mass of these uppermost container tiers. Any containership construction which would allow omission of these hatch covers quite obviously results in several important advantages. First, the cost of the hatch covers is quite substantial. Next, one must appreciate that without the hatch covers, more space and weight allowance is made available for the containers and thus revenue-producing cargo. The above two advantages will also carry over, by the omission of the extra lashings which must be used when carrying containers atop hatch covers. Thirdly, and a most significant advantage, is the time and labor saved in loading and unloading a containership that avoids the necessity of having to use hatch covers over the respective holds. Thousands of dollars are saved during the loading or unloading of a hatchcoverless containership versus the same operation involving the current conventional containership equipped with hatch covers,
Any improvement in containership construction which would enable the elimination of the present hatch covers would obviously provide an immense economical boon to the shipping industry.